The Provence side moved on to 75 points, an unassailable eight-point lead over second-placed Lille with two games left, triggering wild celebrations in the football mad city.
At the Stade Velodrome, Rennes striker Jimmy Briand cancelled out Gabriel Heinze's early opener before the break, but Marseille struck back late in the second half.
"It rewards nine months of hard work," coach Didier Deschamps, who was the last OM captain to lift the national trophy, told French TV channel Orange Sport.
"The first half was not easy for us, with all the expectations weighing on our shoulders."
"I was too young to enjoy Marseille's last titles so tonight I'm going to celebrate hard," Thomas, a 21-year-old fan, told Reuters, his OM scarf wrapped around his neck.
Police officers were patrolling the city's main streets as thousands of fans chanted 'We are the champions, we are the champions'.
The French international, linked with a move to Marseille next season, headed home from Asamoah Gyan's cross after he was left unmarked by Heinze.
Two minutes before the interval, Valbuena found Niang in the box with a defence splitting pass, but keeper Nicolas Douchez dived at the Senegal striker's feet.
Lucho put the result beyond doubt two minutes later after benefiting from excellent work by Ben Arfa on the left flank, the Argentine's curling shot from just outside the box effectively sealing Marseille's ninth French league title.
